This invention relates to electrolytic cells and more particularly to electrolyte separators.
Several types of electrolytic processes and cells are well known in many industrial branches of electrochemistry. "Diaphragm" type cells are often used in the electrolysis of aqueous solutions. This type of cell generally has anolyte and catholyte compartments which are separated by an electrolyte separator. Electrolyte separators prevent interaction of the anolyte and catholyte and separate the gaseous products evolving from each electrode. In the chloralkali industry, for example, cells having electrolyte separators are widely used in the electrolysis of sodium chloride brine to produce sodium hydroxide and chlorine gas.
Electrolyte separators may be rigid, such as porous sheets of plastic or ceramic. Rigid separators, when saturated with electrolyte, are usually electronically conductive while being nonselectively conductive to anions and cations. Separators may also be of the flexible permeable type, such as woven textiles or nonwoven fibrous mattes. Ion exchange membranes are one type of flexible separator. They are essentially permeable to electrolytes while being selectively conductive to either cations or anions.
Flexible separators such as ion exchange membranes have several advantages. These separators are not subject to fracture as are rigid separators. Furthermore, because they are generally thin, they are easy to fabricate and have minimal electrical resistance. As the cost of power to operate electrolyte cells increases, the need for these thin, flexible separators becomes more pronounced.
A major disadvantage of flexible separators is the swelling that usually occurs upon saturation of a separator with an electrolyte. The degree of swelling is dependent upon the concentration and the temperature of the electrolyte. This swelling creates slack in the separator.
Separator distortion is more likely to occur in a slackened separator. Most electrolytic cells employing separators have pressure differentials between the anolyte and the catholyte. Pressure differentials may be caused by density differences, hydrostatic head or kinetic head differences, etc., between the anolyte and catholyte. These forces can cause separator distortion in the form of buckling, weaving or folding. This is particularly true of cells having vertically extending separators. Distortion may lead to a number of malfuntions in the electrolytic cell.
Distortion sometimes is in the form of separator fluttering. This may cause the separator to fail by fatigue at its points of support. Distortion of the separator can block the rise of the product gases and can cause an uneven flow of electrolyte past the electrodes. The distortion may also cause the separator to come into direct contact with an electrode, thereby slowing the electrolysis process at such areas of contact and increasing the current density in other areas. Excessive current densities can result in high "i.sup.2 r" drops and overheating.
Presently, there are no adequate means for eliminating separator swelling in flexible separators. In an attempt to eliminate the aforementioned effects of separator swelling, methods of taking up the slack created by the swelling have been devised. For example, in some cells, the separator is stretched over a rigid frame and tension is applied by wedges inserted along one or more sides of the frame. As the separator swells, the wedges are manually pounded into the frame, thereby taking up the slack in the separator. This technique is sometimes used in metal winning cells with diaphragms of a permeable, woven cloth of man-made fiber. These diaphragms have sufficient rupture strength to withstand the high tension created by the manually inserted wedges.
This wedge tensioning method, however, is unsuitable for stretching relatively thin or weak separators, such as ion exchange membranes, which are used in other types of electrolytic cells. Ion exchange membranes, for example, have relatively low yield strengths which cannot be exceeded. A further problem with this method is that separator swelling varies under different conditions. For example, during a temporary shutdown, electrolyte may be drained from the cell and the separator may cool down, causing the separator to contract. The separator may then rupture if the wedges are not manually loosened to release the tension applied.
Another method proposed for eliminating separator distortion problems is supporting a separator between screens of nonconducting, foraminiferous, structurally strong material. However, screens can create problems by blocking the movement of electrolyte and product gas. They can also increase the distance required between electrodes, thereby causing a higher resistance to current flow.
Reinforcing the separator internally with fibers or woven textiles has also been suggested. This may decrease separator swelling but it does not completely eliminate the problem in cells with large separators. Furthermore, nonconductive fibers added to the separator create additional power losses because of the increased difficulty of ion migration.